The present invention concerns the ground contact system of vehicles, in particular suspension systems and more particularly the guiding of the wheels.
International application WO 01/72572 describes a wheel support device which allows a degree of freedom of the camber of the wheel relative to the suspension elements. This degree of freedom is controlled either actively, for example by a jack as a function of running parameters of the vehicle, or passively by the forces exerted on the wheel.
The “wheel plane” means that plane, related to the wheel, which is perpendicular to the wheel axis and passes through the middle of the tyre. The angular position of the wheel plane relative to the body of the vehicle is defined by two angles, the camber angle and the steering angle. The camber angle of a wheel is the angle which, in a transverse plane perpendicular to the ground, separates the wheel plane from the median plane of the vehicle. This angle is positive when the upper part of the wheel is displaced away from the median plane towards the outside of the vehicle, and in this case one speaks nowadays of “camber” or “positive camber”. Conversely, when the said angle is negative, one speaks of “counter-camber” or “negative camber”. In what follows, “camber” or “camber angle” will be used interchangeably.
The steering angle of a wheel is that angle which, in a horizontal plane parallel to the ground, separates the wheel plane from the median plane of the vehicle.
The camber plane is the plane in which the camber takes place. It is the vertical plane, transverse relative to the vehicle and passing through the centre of the static contact area When the steering angle of the wheel is zero, the camber plane contains the axis of the wheel.
In application WO 01/72572 it is proposed for passive systems that the instantaneous centre of rotation of the camber movement of the wheel relative to the suspension elements should be located below ground level so that the transverse forces acting on the contact area generate a torque which tends to tilt the wheel plane in the desired direction (this instantaneous centre of rotation is called the “first instantaneous centre of rotation” in the document WO 01/72572). However, although under that condition the transverse forces generate a torque in the camber axis which tends to tilt the wheel in the desired direction, the efficiency in terms of camber variation is very different depending on the implemented configurations. In practice, however, the sensitivity of the camber to forces in the contact area is an important criterion. In effect, it is generally sought to design a wheel support and suspension system such that the passive camber variation is predictable, stable and satisfactory in terms of maximum inclination. This is particularly important for very high-performance vehicles intended especially for racing. For such vehicles the search for absolute performance involves optimisation of the longitudinal and transverse grip. This optimisation is only possible if the camber angle of the wheel is at all times close to the ideal for the functioning of the tyre. A camber that is ideal in terms of tyre grip is one that makes it possible to optimise the homogeneity of the pressure distribution in the contact area, i.e. which for example allows compensation of the effect, on the pressure distribution in the contact area, of lateral deformations of the tyre when it is drifting (typically when cornering).